The invention relates to an electric incandescent lamp for series arrangement comprising a translucent envelope, in which a filament is arranged between current-supply wires, which extend through the wall of the envelope to the exterior, the current-supply wires in this envelope being interconnected in an electrically insulating manner by a supporting member and the lamp comprising a shortcircuit switch, which shunts the filament and comprises a vitreous mass or body, in which metal powder is dispersed. Such a lamp is known from GB PS No. 1,077,863.
In such lamps, the shortcircuit switch must be practically impervious to current at the operating voltage, but must become current-conducting at the overvoltage occurring when the filament burns through during operation of the lamp in series arrangement.
In the incandescent lamp known from GB PS No. 1,077,863, the shortcircuit switch is an element consisting of a sintered mixture of glass powder and iron powder, which is fused or sintered to the currentsupply wires. The mixture may be present in a hollow supporting member, may be constructed itself as a supporting member or may be sintered to the outer side of the lamp envelope. This shortcircuit element interconnecting the current-supply wires first acts as an isolator, but should break down upon application of a high electric voltage and hence should shortcircuit the lamp when the filament burns through. The breakdown which should occur in the case of overvoltage in this shortcircuit element is strongly dependent upon not readily controllable quantities, such as mixture of the powders, grain size distribution, fusion or sintering to the current-supply wires, content of moisture during the manufacture of the lamp, etc. Therefore, a reliable shortcircuit switch cannot be readily manufactured.
According to GB PS No. 1,077,863, a shortcircuit element is used for electric incandescent lamps for series arrangement which consists of copper oxide powder and glass powder. This element is also non-conducting in the normal condition and will become conducting only upon application of overvoltage. It has been found in practice that this switch does not operate in a reliable manner either.
This also applies to an incandescent lamp known from GB PS No. 839,160 comprising a shortcircuit element consisting of a paste of copper powder, magnesium oxide and silicone resin, the quantity of magnesium oxide being 18 to 24% by weight of the quantity of copper. This member is also non-conducting, but will become conducting upon application of an overvoltage.